As asserted in Lundback S., "Cardiac Pumping and Function of the Ventricular Septum", Stockholm, 1986, the pumping and regulation of the human heart take place in a manner which is at variance with the prevalent view.
According to the cited publication, the healthY heart performs its pumping action without substantially changing its outer shape and volume.
More particularly, during ventricular systole (the active, expulsive phase of the heart cycle) the so-called valve plane, that is, the plane containing the atrioventricular heart valves, is drawn towards the heart apex and forces the blood contained in the ventricles into the pulmonic and systemic circulation, and at the same time blood is drawn into the atria as a consequence of the movement of the valve plane.
During ventricular diastole, the phase of the heart cycle in which the heart muscle is relaxed, the valve plane is returned to the initial position under the influence of the momentum which is imparted to the inflowing blood as a consequence of the downward movement of the valve plane during ventricular systole.
As is also asserted in the publication (on the basis of the finding that the outer volume and shape of the heart are substantially constant over the heart cycle), the ability of the heart to change the relative volumetric capacities of the right and left ventricles is attributable only to the common ventricular wall, the ventricular septum, namely by virtue of its flexibility in the relaxed state of the heart. During ventricular systole the ventricular septum together with the rest of the left ventricular musculature always assumes a circular cross-sectional configuration and takes a distinct position independentlY of its shape and position during diastole. This is so, because during ventricular systole the pressure in the left ventricle is always higher than the pressure in the right ventricle. If the configuration and position of the ventricular septum during diastole, the relaxed state, are different from the configuration and position during systole, the active state, the ventricular septum, acting like a diaphragm pump, therefore provides an increased stroke volume for one ventricle and a correspondingly reduced stroke volume for the other ventricle. In this way, the ventricular septum accomplishes a double-acting regulation to maintain the balance between the two branches of the circulatory system (the pulmonary circulation and the systemic circulation).
Many advantages are attributable to this mode of work and regulation of the heart, and the realization of this mode of operation answers questions concerning the physiology of the heart which have not until now been answered consistently.
For the heart to be able to perform its pumping and regulation with constant outer shape and volume, it is necessary that the muscular power of the ventricular septum is not substantially reduced in comparison with the rest of the musculature of the left ventricle, as may be the case for example when an infarction has been inflicted on the ventricular septum. As a consequence of such a damage, the ventricular septum during ventricular systole becomes unable both to withstand the pressure in the left ventricle and to perform a proper regulatory function. Therefore, the ventricular septum will yield to the pressure in the left ventricle and provide an abnormally increased stroke volume on the right side of the heart and a corresponding reduction of the stroke volume on the left side. Consequently, more blood is pumped into the pulmonary circulation than into the systemic circulation. This, in turn, because of the impaired or lost regulatory function of the ventricular septum, causes an accumulation of blood in the lungs resulting in pulmonary edema. Such a course of events is life-threatening.
However, experiments carried out on animals with the heart working in the opened thorax, that is, surrounded by air, have shown that simulated infarctions inflicted on the ventricular septum do not have the disastrous consequences indicated above.
Under the conditions in which the experiments were carried out, the heart works as a different type of displacement pump, namely, as a pump doing its work with varying outer volume and shape. This also means that the ventricles under diastole can change their volumes not only as a result of displacement of the ventricular septum, but also as a result of displacement of the rest of the ventricular walls. During systole the ventricles can therefore provide different stroke volumes without influencing one another as much. This type of pumping and regulation is consistent with the prevalent view of the way the heart works when it is enclosed in the body.